(1) Field of the Invention
This invention relates to a digital signal processor and a method for producing harmonic sound.
(2) Description of Related Art
In compressed music signals such as MP3 or WMA, a high frequency range to which a human hardly listen is cut for reducing its file size. Therefore, there is a problem that sound is deteriorated by compression of music signals. Accordingly, a harmonic sound generator is proposed for restoring the high frequency range by generating harmonic sound from the music signals.
A conventional harmonic sound generator uses a compressor having an input-output characteristic shown in FIG. 1. As shown in FIG. 1, when an input signal is less than a specific value A, the compressor outputs linearly, and when the input signal is more than the specific value A, the compressor outputs the specific value A. Accordingly, When a sine wave music signal shown in FIG. 2A is inputted into the compressor, the compressor outputs a music signal of which range over the specific value A is distorted as shown in FIG. 2B. FIG. 3 shows a relationship between a frequency and a signal level of the music signal shown in FIG. 2B. As it is clear from FIG. 3, the music signal shown in FIG. 2B includes harmonic sound components 2f1, 3f1, 4f1, and the like in addition to a frequency f1 of the original music signal.
Further, using a DSP (digital signal processor) instead of the compressor is also proposed to generate the harmonic sound by converting the signal level of the music signal according to a non-linear function the same as the input-output characteristic shown in FIG. 1 (See Patent Document 1).
Patent Document 1: Japanese Published Patent Application No. H05-6177
However, there is a problem that according to a method for generating harmonic sound as described above, the harmonic sound cannot be generated on the basis of the music signal of which signal level is less than the specific value A. There is also a problem that a non-linear input-output device such as the compressor is necessary, thereby a scale of a circuit is increased.
Further, according to the conventional method for generating harmonic sound as described above, the harmonic sound is generated on the basis of all the frequencies included in the music signal. Therefore, there is a problem that it is impossible that harmonic sound on the basis of only a vocal frequency range is generated to emphasize vocal sound.
Accordingly, an object of the present invention is to provide a digital signal processor and a method for generating harmonic sound so as to surely and simply generate harmonic sound on the basis of even a music signal with a small signal level.
Another object of the present invention is to provide a digital signal processor and a method for generating harmonic sound so as to emphasize a music signal of a specific frequency range.